A variety of solar energy utilization schemes have been used, with varying degrees of success, throughout recorded history. However, abundant fuel supplies have usually limited the use of this diffuse and often capricious energy source. On the other hand, it is apparent that increasing energy requirements may well make it necessary to resort to solar energy for a substantial portion of our total energy needs.
For solar energy utilization systems of greater sophistication than simple greenhouses and the like, both air and water have been proposed as the primary heat transfer medium. Water, with or without added antifreeze, offers the advantage of high density and high specific heat and hence has been employed in the majority of solar energy systems. These systems, however, present substantial potential freezeup problems. If antifreeze is employed there may be contamination difficulties at any interface with a potable water supply. Corrosion (rust, scale, etc.) presents another substantial problem. Furthermore, the economic advantages attendant upon the efficiency of water as a heat transfer medium may be offset by the cost of electrical controls for the pumps and control valves necessary to the system.
Although air, as a primary heat transfer medium, has the disadvantage of low density and low specific heat, it effectively minimizes and even eliminates many of these problems. Thus, with an air-based solar energy system, there are no problems of freezing or contamination of a water supply, and few or no difficulties with corrosion. The requirements for electrical controls may still be substantial, from an economic standpoint, but these can also be effectively minimized as described below.
Solar energy systems can be roughly categorized into three types with respect to end use temperature:
low (80.degree. to 100.degree.) PA2 medium (100.degree. to 140.degree.) PA2 high (above 140.degree.).
Low temperature recovery systems are most suitable for industrial pre-heating, swimming pool heating, and greenhouse applications. They overlap to some extent with medium temperature recovery systems, which are most practical for use with domestic hot water and space heating. Systems operating in the high temperature range are most frequently employed for steam generation and the more exotic uses such as metallurgical smelting. The present invention is principally concerned with the low and medium end use temperature ranges.
For low and medium end use temperatures, so-called "flat plate collectors" are usually preferred as the initial energy collection devices. In general, a flat plate collector includes a relatively planar enclosed solar radiation absorbing surface with a transparent glazing window, the absorbing surface being a part of a heat exchanger through which a heat transfer fluid is circulated. The collector is usually oriented to face toward the sun (south in the northern hemisphere) and at an angle to the horizontal such that its surface is normal to the incident solar radiation for mid-year (equinox) conditions. High temperature systems may shift the collectors to track movements of the sun, but such tracking systems are economically infeasible for low to moderate temperature installations. The fixed flat plate collectors usually employed in low and medium end use temperature systems intercept considerably less direct solar flux because, even under the best of conditions, they are accurately aligned with the sun for only a short portion of the solar day.
In the evaluation of solar energy systems, a frequently used measurement of efficiency is the ratio of actual recoverable heat energy from the collector to the known incident solar flux for optimum (noon) conditions. For example, if a given collector intercepts solar flux of 200 BTU per hour per square foot of collector surface and is capable of delivering 100 BTU per hour to an external energy utilization means for end use, the efficiency is said to be fifty percent. From a practical standpoint, there are several important variables having a direct bearing on actual operating efficiency of a solar energy system, and some of those variables are not accounted for in the conventional efficiency comparison.
Two of those variables are the absolute value of incident solar flux and the temperature difference (.DELTA.T) between the absorption element in the collector and the ambient air. Regardless of the construction employed for the collector, the efficiency of this portion of the system is directly proportional to the incident solar flux and is inversely proportional to the temperature difference. These two factors are pretty well accounted for by conventional efficiency measurements for actual noon-time conditions.
Another variable factor that affects efficiency is thermal loss from conduction or convection, at the collectors, and is generally dealt with by means of insulation. Good insulating practice can provide substantial improvements in efficiency. The glazing on the collector contributes worthwhile, and in some instances essential, insulation by providing a dead air space between the heat exchange portion of the collector and the ambient air. The type of absorption surface used in the collector may have a substantial bearing on its overall efficiency. Some surface coatings for the heat exchanger portion of the collector show a substantial increase in the ratio of absorption to reflection. On the other hand, for systems of small and moderate size, particularly those intended for domestic use, the use of high absorption coatings may entail costs that are excessive in relation to the improvements obtained.
A factor of considerable importance in the construction of the collector panels is the glazing. All glazing materials absorb some of the incident solar radiation, as much as ten percent in many cases. However, by far the most important loss from the glazing fixed flat plate collectors is due to the reflection from the glazing surface. On any smooth surface, the ratio of reflection to transmission increases rapidly as the angle of incidence of the solar radiation departs from the normal. For example, the reflection/transmission ratio of window glass is approximately five to one at an hour angle of three hours (45.degree.). This situation continues to deteriorate as the incident angle becomes more acute. In fact, in terms of a solar day this factor alone may predominate over all of the others combined.
Reflection from the glazing surface has a critical effect on efficiency. As previously noted, "efficiency" data is normally obtained with the collector surface normal to the incident radiation. Though useful as a rough comparison, this determination of little value in terms of system design, sizing and performance evaluations and calculations because of the variations in the relative position of the sun, with low to moderate angles of incidence during most of the solar day. Consequently, much of the solar flux is reflected back into space and only a limited proportion reaches the heat absorber in the collector. This reduction is further compounded by decreases in solar flux associated with changes in the solar year. Thus, the net efficiency of a flat plate collector is materially reduced as compared with the normal determinations of efficiency, and may in fact be as low as 5% or 10% for a collector that tests 50% to 70% under standard idealized conditions. This is an area in which large gains may be made in solar energy recovery.
For effective use of air as a heat transfer medium, the movement of large volumes of air are essential. In most known solar energy systems using air as a primary heat transfer medium, these large volumes are moved at relatively low pressures, requiring large and expensive duct systems. It has also been customary, in solar energy systems using air as a primary heat transfer medium, to employ that air directly for end use purposes, as in space heating. In consequence, air used as the heat transfer medium becomes contaminated with grease, dust, lint, smoke, and the like. This frequently results in a material reduction in overall efficiency of the system and, ultimately, in a necessity for expensive cleaning.